As the offshore drilling and production industry moves into deeper waters, the need for subsea wellhead equipment, such as christmas trees and control valves, which can be operated remotely in depths beyond those at which divers can work becomes increasingly greater. Further, automatic or remotely actuated control valves such as are used on subsea wellheads require some means for manually or mechanically overriding the valve in the event the hydraulic actuator should fail to function properly. Additionally, it is desirable to have an override mechanism for moving the valve to the open position with a minimum number of seals to thereby enhance the inherent reliability of the system. It is therefore an object of the present invention to provide a new and improved gate valve with an emergency override having a hermetic seal to provide a means for overriding the valve and requiring a minimum number of seals to protect the override mechanism.